Radiographic images such typically as X-ray images are widely employed in various fields, for example, in the medical diagnosis. To obtain the X-ray image, radio-photographic method is principally utilized in which radiation penetrated through a subject is irradiated to a phosphor layer, also called as a phosphor screen, and light emitted from the phosphor layer is irradiated to silver halide photographic light-sensitive material and then the light-sensitive material is developed to obtain a visible image. Recently, however, new method for directly obtaining the image from the phosphor layer is proposed instead of the image forming method by the light-sensitive material.
In such the method, radiation penetrated through the subject is absorbed by a phosphor and then the phosphor is stimulated by light or thermal energy so as that the energy accumulated in the phosphor by absorbing the radiation is released as fluorescence and the fluorescence is detected and imaged. In concrete, the method is a radiation image conversion method utilizing a stimulable phosphor such as that described in U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection, hereinafter referred to as Japanese Patent O.P.I. Publication, No. 55-12144.
In this method, a radiation image conversion panel containing the stimulable phosphor is utilized. In detail, the radiation image conversion panel is exposed to radiation penetrated through the subject so that the radiation energy corresponding to the radiation transmission of each parts of the subject is accumulated in the stimulable phosphor, and then the energy of the radiation accumulated in the phosphor is emitted as light by time serially stimulating the phosphor by stimulating electromagnetic radiation such as visible light and infrared rays. The signal according to the variation of the emitted light is readout, for example, as electric signals by photo-electric conversion, and the electric signals are reproduced as a visible image on a usual image recording medium such as the light sensitive material or an image displaying apparatus typically such as a CRT.
The foregoing reproduction method for reproducing the radiographic image record has an advantage such as that a radiographic image with abundant information by an exposure amount of radiation considerably smaller than that to be required in the radio-photographic method employing a combination of the radiographic light-sensitive material and an intensifying screen.
On the outer surface of the stimulable phosphor layer, namely the surface not to be faced to the support, a protective layer for protecting the phosphor layer from a chemical deterioration or a physical impact. As the protective layer, the followings have been known: one formed by coating on the phosphor layer a solution composed of an organic polymer such as a cellulose derivative and poly(methyl methacrylate) dissolved in a suitable solvent; one formed by adhering a separately prepared protective layer forming sheet or film composed of an organic film such as polypropylene and poly(ethylene terephthalate) or a glass plate on the phosphor layer by a suitable adhering agent; and one formed by depositing an inorganic compound by a method such as vapor deposition.
The stimulable phosphor is a phosphor which emits light when irradiated by stimulating light after exposed to radiation. The phosphor emitting light of from 300 to 500 nm by stimulating light of from 400 to 900 nm is usually employed in practice. Examples of the stimulable phosphor employed hitherto in the radiation image conversion panel are alkali-earth metal fluoride-halide compound type phosphors each activated by a rare-earth element described on Japanese Patent O.P.I. Publication Nos. 55-12145, 55-160078, 56-74175, 56-116777, 57-23673, 57-23675, 58-206678, 59-27289, 59-27980, 59-56479 and 59-56480; di-valent europium-activated alkali-earth metal halide type phosphors described in 59-75200, 60-84381, 60-106752, 60-166379, 60-221483, 60-228592, 60-228593, 61-23679, 61-120882, 61-120883, 61-120885, 61-235486 and 61-235487; rare-earth element-activated oxohalide phosphors described in Japanese Patent O.P.I. Publication No. 59-12144; cerium-activated tri-valent metal oxohalide phosphors described in Japanese Patent O.P.I. Publication No. 58-69281; bismuth-activated alkali halide type phosphors described in Japanese Patent O.P.I. Publication No. 60-70484; di-valent europium-activated alkali-earth metal halo-phosphate phosphors described in Japanese Patent O.P.I. Publication Nos. 60-141783 and 60-157100; di-valent europium-activated alkali-earth halo-borate phosphors described in Japanese Patent O.P.I. Publication No. 60-157099; di-valent europium-activated alkali-earth hydrogen-halide phosphors described in Japanese Patent O.P.I. Publication No. 60-217354; cerium-activated rare-earth composite halide phosphors described in Japanese Patent O.P.I. Publication Nos. 61-21173 and 61-21182; cerium-activated rare-earth halo-phosphate phosphors described in Japanese Patent O.P.I. Publication No. 61-40390; di-valent europium-activated cerium rubidium phosphors described in Japanese Patent O.P.I. Publication No. 60-78151; di-valent europium-activated halogen phosphor described in Japanese Patent O.P.I. Publication No. 60-78153; and rare-earth metal-activated tetradecahedron alkali-earth metal fluoride-halide type phosphors precipitated from a liquid phase described in Japanese Patent O.P.I. Publication No. 7-233369.
Among the above-described stimulable phosphors, the di-valent europium-activated iodide-containing alkali-earth metal fluoride-halide type phosphors, the di-valent europium-activated iodide-containing alkali-earth metal halide type phosphors, the rare-earth metal-activated iodide-containing rare-earth metal oxohalide type phosphors, and the bismuth-activated iodide-containing alkali metal halide type phosphors each emit high luminance light.
It is one of the advantages of the radiation image conversion panel employing that the panel can be repeatedly usable because such the phosphor emits by scanning by the stimulating light and radiographic image can be re-accumulated thereafter. Such the method is also advantageous from the viewpoint of the resources protection and the economical efficiency since the radiation image conversion panel can be repeatedly usable contrary to that the radiographic light-sensitive material is consumed every once of the photographing in the usual radiographic method.
As described above, though the radiographic image recording-reproducing method has various advantages, it is demanded to the image conversion panel to be used in such the method that the sensitivity and the image quality such as the sharpness and the graininess are as higher as possible.
In the performance of the method, the radiation image conversion panel is repeatedly employed in the circle of exposure to radiation for recording the radiographic image, irradiation of the stimulating light for reading the recorded radiographic image and irradiation of erasing light for erasing the remained radiographic image. In a cassette type panel, the panel is conveyed through each steps by a conveying means such as a belt and a roller to perform such the repeatedly use of the panel. On the occasion of such the operation, the temperature of the interior of the apparatus is made higher than the room temperature by the presence of the light source and the driving system. In the case of an exclusive type apparatus, the temperature interior of the apparatus tends to be raised higher than that of the cassette type apparatus by heating by the light source in the course of continuous image taking even though there is no panel conveying means in the apparatus. It has been known that the unevenness of image density is degraded when heat is applied. Consequently, high resistivity against heat is required to the panel.
On the other hand, it has been considered that the sharpness of the image obtained by the radiation image conversion panel is higher when the thickness of the protective layer is thinner. However, the durability of the conversion panel tends to be lowered because the scratches and the cracks on the protective layer surface tend to be formed in the course of the repeatedly use when the thickness of the protective layer is thin. Therefore, suitable materials are elected so that the functions as the protective layer such as anti-scratch property and the conveying suitability can be maintained even when the thickness of the protective layer is made as small as possible for inhibiting the degradation of the sharpness. For example, the use of a material having a high strength and transparency such as poly(ethylene terephthalate), combination of plural kinds of resins, coating of a layer of a composition containing fluorine-containing resin having high anti-scratch ability and the use of multi-layered protective layer are applied.
However, the thickness of the protective layer should be increased in the usual technology to obtain the durability for prolonged period in the apparatus so that degradation of the sharpness is resulted.
Problems that unevenness of the density other than the density variation caused by the subject occurs when a film of polypropylene, poly(ethylene terephthalate) or poly(ethylene naphthalate) is used as protective layer, or the density unevenness caused by the coating of the resin composition containing fluorinated resin occurs. As the countermeasure to such the density unevenness, Japanese Patent O.P.I. Publication No. 59-42500 and Japanese Examined Patent Publication No. 1-57759 discloses a measure to disappear the density unevenness by raising the haze ratio of the protective layer. However, a drawback is caused that the increasing of the haze ratio results further lowering of the sharpness.
Consequently, a method described in, for example, Japanese Patent O.P.I. Publication No. 10-82899, in which the protective layer is constituted by a plastic film and a fluorinated resin-containing resin composition which contains light scattering fine particles coated on the film, and a method described in, for example, Japanese Patent O.P.I Publication No. 2002-122698, in which the protective film has a stimulating light absorbing layer and specified surface roughness, are disclosed. However, the improvement of the sharpness is insufficient and the compatibility of high sharpness and low density unevenness cannot be satisfied yet.